1. Technical Field
The present invention generally relates to ceramic materials. More particularly, it relates to ceramic materials used in insulators of spark plugs.
2. Related Art
Spark plugs, glow plugs, and other such devices used in internal combustion engines are subjected to high temperature environments in the region of about 1,000° C. In general, a spark plug is a device that extends into a combustion chamber of an internal combustion engine and produces a spark to ignite a combustible mixture of air and fuel therein. Specifically, a spark plug typically includes a cylindrical metal shell having external threads that screw into a portion of the engine and further having a hook-shaped ground electrode attached thereto at a firing end of the spark plug. A cylindrical insulator is disposed partially within the metal shell, and extends axially beyond the metal shell toward the firing end. A conductive terminal is disposed within a cylindrical insulator at a terminal end of the spark plug opposite the firing end. At the firing end, a cylindrical center electrode is disposed within the insulator and projects axially out of the insulator toward the ground electrode, whereby a spark plug gap is defined between the electrodes.
In operation, ignition voltage pulses of up to about 40,000 volts are applied through the spark plug to the center electrode, thereby causing a spark to jump the gap between the center and ground electrodes. The spark ignites an air and fuel mixture within the combustion chamber to create high temperature combustion to power the engine. Unfortunately, the high voltage and high temperature environment within the combustion chamber can degrade the different components of the spark plug and, over time, can negatively affect the characteristics of these components, thus altering the intensity of the ignition pulse over time and ultimately degrading the quality of the spark. In particular, degradation of the ceramic insulator can lead to dielectric puncture through the insulator which establishes an alternative electrical path through the insulator, and consequently the spark may not reliably jump the gap between the center and ground electrodes. The quality of the spark affects the ignition of the mixture of the air and fuel (i.e., the combustion efficiency, combustion temperature, combustion products) and, thus, the power output and fuel efficiency performance of the engine and the nature of the emissions produced by the combustion of the air and fuel. Due to an increasing emphasis on regulation of emissions from motor vehicles, it is desirable to maintain a high quality spark to provide constant, consistent engine performance and emission quality. The quality of the spark is determined by several factors including the material composition of the ceramic insulator material.
The ceramic insulator materials used for the cylindrical insulator are dielectric materials. Dielectric strength is generally defined as the maximum electric field which can be applied to the material without causing breakdown or electrical puncture thereof. For a device such as a spark plug, dielectric strength is generally measured in kilovolts per mil (kV/mil). For a given spark plug design, the insulator dimensions are fixed, thus, dielectric strength is frequently expressed as a breakdown voltage in kV, rather than in kV/mil. A typical value for spark plug dielectric strength for a standard spark plug design used in many applications is on the order of about 40 kV at room temperature. Dielectric strength of the ceramic insulators used in spark plugs is also a function of temperature. High temperatures tend to cause an increase in the mobility of certain ions in these ceramic materials, allowing a small leakage current to pass through the ceramic. This leakage of current leads to localized heating which gradually degrades the resistance of the material to dielectric puncture. It has been observed that resistance of ceramic materials to dielectric breakdown tends to decrease over the life of a spark plug due to thermal stress on the spark plug cycling under an applied electric field and due to attendant thermal-electrical fatigue thereof. The exact nature of the microstructural and/or compositional changes are not completely understood, but are believed to be associated with localized heating to temperatures sufficient to bring about partial melting of the ceramic material.
Shunt resistance is another measurable property of ceramics, particularly for those used in spark plugs, and is a measure of the electrical resistance of the material which is generally measured in megaohms. A typical value for spark plug shunt resistance is on the order of about 75 to 125 megaohms at an operating temperature of about 1000 degrees Fahrenheit. Shunt resistance is typically measured on a spark plug as an electrical resistance imposed by or associated with the ceramic insulator—measured between the center electrode and metal shell of the spark plug. In other words, shunt resistance is indicative of the amount of current leakage through the ceramic insulator between the center electrode and metal shell or housing. Whereas dielectric breakdown tends to be a sudden, discrete event, low shunt resistance tends to take the form of a continuous, parasitic loss, which may ultimately result in an increased likelihood of catastrophic dielectric failure after the spark plug has been in used for an extended period of time.
A breakdown in dielectric strength and/or shunt resistance ultimately leads to shunting of the spark plug. Shunting of the spark plug is a condition in which an undesirable parallel conductive path is established between the center electrode and the metal casing in addition to the path across the spark gap between the center electrode and the ground electrode. Shunting generally has an adverse affect on the quality of the spark generated by the spark plug. In the case of shunting due to dielectric breakdown the affect is generally catastrophic. However, in the case of shunting caused by diminished or insufficient shunt resistance, the affect is may simply degrade the spark performance of the plug, and consequently the performance of the engine, as described above, or may result in an increased likelihood of catastrophic dielectric loss after the spark plug has been in use for an extended period of time.
Therefore, it would be desirable to produce a spark plug using an improved ceramic insulator material with high shunt resistance that is less susceptible to a breakdown in dielectric strength for extended periods of time at high voltages and high temperatures and, thus, less susceptible to shunting conditions in the spark plug, in order to promote generation of a quality spark and enhanced engine performance.